I think it's actually a diastolic (late) murmur or I guess atrial systole murmur. It's on FA 2009 pg 248. Both S3 and S4 happen during diastolic ventricular filling. S3 is heard immediately after the isovolumetric relaxation in diastole when the mitral valve opens. It's due to the increased filling pressure of the LV. I think the confusing part is that it's a systolic dysfunction, but causes a diastolic murmur. The murmur is heard due to the filling of the dilated LV, while the systolic dysfunction occurs due to pump failure caused by a weakened/dilated LV and the increased preload in the LV. S4 is heard at the end of diastole right before the mitral valve closed, due to the atrial kick caused by the outflow tract obstruction caused by the LA pushing against the hypertrophic LV.
- Dilated Cardiomyopathy = S3 (heard in early diastole), Systolic Df -->
PUMP FAILURE
- Hypertrophic Cardiomyopathy = S4 (heard in late diastole), Diastolic Df-->
OUTFLOW TRACT OBSTRUCTION